ABSTRACT There is substantial evidence linking the gastrointestinal (GI) microbiota and obesity. Germ free (GF) mice do not gain weight when fed a high fat (HF) diet. Microbiota composition rapidly changes in response to HF feeding and colonization of GF animals with an ?obese? microbiota results in recapitulation of the donor phenotype. This data suggest that an unfavorable microbiota is sufficient to cause obesity, however the mechanisms and pathways remain unclear. Excessive energy intake is the main cause for obesity. Food intake is regulated by homeostatic and hedonic cues. Hedonic eating refers to consumption of food ?for pleasure?, in the absence of or beyond energy needs and is linked to reward signaling in the forebrain. Food consumption, particularly HF food, leads to the release of dopamine in the brain, hyper- and hyposensitivity of this system have been linked to abnormal weight gain. The microbiota has previously been shown to alter neural development and gene expression in the brain. However the potential influence of the microbiota on reward signaling and appetitive eating has yet to be studied. Using GF animals colonized with microbiota from lean or obese donors we will test the hypothesis that the microbiota reduces dopamine release in the nucleus accumbens to increase motivation and preference for fat. In order to aid in the development of microbiota- based therapies, it is necessary to understand the route by which the microbiota communicates to the brain. There is evidence that microbiota to brain signaling is relayed by vagal afferents innervating the GI tract but our understanding of the pathway is limited, partially because traditional techniques, such as vagotomy and capsaicin, lack specificity and can indirectly alter microbiota composition. In the second aim of this proposal, we will use a ribosome inactivating protein to ablate vagal afferent signaling in colonized animals. We aim to demonstrate that microbiota to brain communication is vagally mediated. Knowledge from this proposal will support the development of microbiota-based therapies aimed at food addiction and weight loss. Microbiota and vagal signaling could be more easily manipulated with fewer side effects than central targets.